Lightning arrester

ABSTRACT

The disclosed gap structure includes two opposed electrodes and a triggering electrode positioned between them to define two gaps with them but not to divide a main electric arc developed between the opposite electrodes. Two impedance elements are connected across the gaps respectively for purpose of voltage division so that, upon applying voltages of the same waveform across the gaps respectively, a ratio of a sparkover voltage across one of the gaps to that across the other gaps ranges from 0.5 to 2.0. This causes the sparkover voltage-to-time characteristic of the gap structure to be flat.

United States Patent Nitta et a1.

[54] LIGHTNING ARRESTER [72] Inventors: Tohei Nitta; Yoshikazu Shibuya; Yuklo Fujiwara, all of Amagasaki, Japan [73] Assignee:

v [52] US. Cl ..3l7/62, 317/69, 313/197,

313/308, 315/330 51 mm. .nozn 1/04 as FieldofSearch ..3l7/62,69,61.5;313/D1G.5,

[ July 25, 1972 1,477,304 12/1923 Allcutt ..315/234 Primary Examiner-J. D. Miller Assistant Examiner-Harvey Fendelman Attorney-Robert E. Burns and Emmanuel J. Lobato [5 7] ABSTRACT The disclosed gap structure includes two opposed electrodes and a triggering electrode positioned between them to define two gaps with them but not to divide a main electric are developed between the opposite electrodes. Two impedance elements are connected across the gaps respectively for purpose of voltage division so that, upon applying voltages of the same waveform across the gaps respectively, a ratio of a sparkover voltage across one of the gaps to that across the other gaps ranges from 0.5 to 2.0. This causes the sparkover voltage- [56] References Cited to-time characteristic of the gap structure to be flat.

UNITED STATES PATENTS 2 Claims, 9 Drawing Figures 1,477,303 12/1923 Allcutt .313/197 VOLTAGE PAIENIEBmzsmz 3.679.939

SHEET 1 OF 2 DJ 0 1 J O a: ll] O X I E U) o l l 1 0 TIME 0.5 I0 I00 1000 0:

TIME BETWEEN APPLICATION OF IMPULSE VOLTAGE AND INITIATION OF DISCHARGE in 5 F/6.3 9 FIG. 4

I 3 A G A G C 4 IMPED- ANCE z. IMPED- IMPED- B ANCE z. I ANCE 2,

TIME DELAY in #8 VOLTAGE RATIO PATENTEDJUL 25 m2 sum 2 or 2 IMPEDANCE IMPEDANCE FIG. 7

LL] 3 so t] 0 L2 \L m 5 o 20 L x l l l a: o I I0 I00 I000 E 0') l I l I TIME in s 0 I I0 I00 I000 TIME in 115 LIGHTNING ARRESTER BACKGROUND OF THE INVENTION This invention relates to a lightning arrester device including, as an arc-extinguishing medium, a gas high in electrically negative property such as sulfur hexafluoride (SF gas, and more particularly to improvements in a discharging gap structure for use in such a device.

Discharging gap structures for use in lightning arrester devices are required to be discharged at a substantially equal sparkover voltage in response to any abnonnal voltage applied thereacross, regardless of whether the wave front thereof is steep or slow, for the purpose of effectively protecting the associated equipments from the abnormal voltage. To meet that requirement, there have been already proposed discharging gap structures of the type including, for example, a pair of spherical electrodes disposed in spaced opposite relationship, a triggering electrode disposed adjacent one of the spherical electrodes, and one impedance element connecting the triggering electrode to each of the spherical electrodes. This type of discharging gap structures has been extremely effectively operated in the arc-extinguishing medium of the prior art type formed of air or nitrogen. However, such discharging gap structures adapted to be operated in the arc-extinguishing medium formed of a gas high in electrically negative property such as sulfur hexafluoride (SF gas are disadvantageous in that steep wave front sparkover voltages can not decrease because in such a gas a spark breakdown is apt to be formed with a long time lag.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention to provide in a lighting arrester device including an arc-extinguishing medium formed of a gas high in electrically negative property, an improved discharging gap structure substantially flat in sparkover voltage-to-time characteristic.

The invention accomplishes this object by the provision of a discharging gap structure for use in a lightning arrester device including an arc-extinguishing medium formed of an electrically negative gas, said gap structure comprising a pair of main discharge electrodes disposed in spaced opposed relationship, and a triggering electrode disposed between the main discharge electrodes to define a pair of discharging gaps with the main discharge electrodes, characterized in that the triggering electrode is located at such a position that, upon applying voltages of the same waveform across the discharging gaps respectively, a ratio of a sparkover voltage across one of the discharging gaps to that over the other gap ranges from 0.5 to 2.0 and that the triggering electrode permits the main discharge electrodes to form a main single electric arc therebetween while the triggering electrode does not divide the main electric arc.

Preferably, a pair of voltage dividing elements may be connected in series with each other and across the discharging gaps respectively and have such magnitudes of impedance that a steep wave front voltage is substantially entirely applied across one of the discharging gaps while a slow wave front voltage is divided into a pair Of voltage portions as determined by the ratio of the sparkover voltage across one of the discharging gaps to that across the other gap, the voltage portions being applied across the discharging gaps respectively.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a graph illustrating waveforms of voltage applied across a discharging gap structure;

FIG. 2 is a graph illustrating the sparkover voltage-to-time characteristics for sulfur hexafluoride SF gas;

FIGS. 3 and 4 are schematic diagrams of discharging gap structures constructed in accordance with the principles of the prior art;

FIG. 5 is a graph illustrating the sparkover voltage-to-time characteristics of the arrangement shown in FIG. 4 for different gases;

FIG. 6 is a schematic diagram of a discharging gap structure useful in explaining the principles of the invention;

FIG. 7 is a schematic diagram of a discharging gap structure constructed in accordance with the principles of the invention;

FIG. 8 is a graph illustrating the results of a calculation made with the voltage dividing characteristic of the gap structure shown in FIG. 7; and I FIG. 9 is a curves plotting sparkover voltage against time for the discharging gap structures shown in FIG. 7.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawing wherein the axis of represents time, an impulse voltage is shown as having a waveform E Assuming that the application of the waveform E across a discharging gap structure (not shown) causes an electric discharge across the gap structure at a time point or a point P on the wave front of the waveform E the discharge voltage has a waveform as shown at thick line E in FIG. 1. A voltage at which an electric discharge is initiated across a discharging gap or a sparkover voltage thereacross generally depends upon a steepness of wave front of a voltage waveform applied across the gap. The relationship between the sparkover voltage across the gap and a time between the application of the voltage waveform across the gap and the initiation of an electric discharge thereacross is represented by a voltage-to-time characteristic (which is abbreviated hereinafter to V-T characteristic) such as shown by the reference character A" or B" in FIG. 2 wherein the axis of ordinates represents sparkover voltage in kilovolts and the axis of abscissas represents a time between the application of an impulse voltage across the gap and the occurrence of the resulting electric discharge thereacross in microseconds. It is desirable to impart the flat V-T characteristic to discharging gap structures for use in lightning arrester devices.

For discharging gap structures disposed in an arc-extinguishing medium formed of nitrogen or air, a flat V-T characteristic can be relatively easily obtained but with the arc-extinguishing medium formed of an electrically negative gas excellent in arc-extinguishing property, for example, sulfur hexafluoride (SF gas, the sparkover voltage is appreciately higher for steep wave front voltages than for slow wave front voltages. This is because an interval of time up to the formation of spark breakdown after the application of the voltage across the gap is very long for electrically negative gases. That interval of time depends upon the configuration of the discharge electrodes involved. In FIG. 2 the reference character A designates typical V-T characteristics for uniform gaps or with a pair of electrodes having their round ends disposed in spaced opposite relationship and the reference character 8" designates typical V-T characteristics for non-uniform gaps or with a pair of electrodes having their sharpened tips disposed in spaced opposite relationship. Those V-T characteristics where obtained with the electrodes disposed in an atmospher of SF,, gas under 1 absolute atmosphere while the discharging gap therebetween was of 4 millimeters. The end portion of the electrode had a radius of curvature of 3 millimeters for the V-T characteristics A and of 0.1 millimeter for the V-T characteristics B.

From FIG. 2 it is seen that, with the discharging gap size remaining unchanged, steep wave front voltages have resulted in sparkover voltage scarcely varied between for the uniform gap and for the non-uniform gap whereas slow front wave voltages have resulted in sparkover voltages appreciably less for the non-uniform gap than for the uniform gap. However, for lightning arrester devices including an arc-extinguishing medium formed of an electrically negative gas such as SF gas, it is desirable to utilize the non-uniform discharge gap in view of the standpoint of the arc interrupting characteristic. In that event, thedischarge characteristics are required to be greatly improved so as to render the V-T characteristic flat.

In order to improve the V-T characteristic associated with the electric discharge across discharging gap structures, there has been previously proposed an arrangement as shown in FIG. 3. The arrangement illustrated comprises three spherical electrodes-l, 2 and 3 disposed in-aligned relationship to form a pair of discharging gaps G of substantial equal length therebetween, and one impedance element 2 or 2 connected across each of the gaps G or across a terminal A or C conn'ected to the electrode 1 or 3 and an intermediate terminal B connected to the intermediate electrode 2. The magnitudes of the impedance elements 2 and Z can be selected so that, upon applying a slow wave front voltage across the terminals 7 A and C, that portion of the .voltage developed across the ter- C both the voltage portions are not equal in magnitude to each other. Under these circumstances, a sparkover voltage across the terminals A and C is equal to twice that across the gap G for a slow wave front voltage. However, for a steep wave front voltage the sparkover voltage across the terminals A and C can be less than twice that across the gap G. Therefore it can be considered that, by properly selecting the pair of the volt age dividing impedance elements Z and 2 connected in parallel to the discharging gaps G respectively, the overall discharge characteristic is capable of being theoretically controlled at will. For example, if for any steep wave front voltage only one of the discharging gap has applied thereacross the entire magnitude of the voltage then the sparkover voltage across the arrangement of FIG. 3 can be theoretically halved.

However the arrangement of FIG. 3 has actually been difficult to improve much the discharge characteristics thereof for the reason that the arrangement is electrically discharged with a time delay. If a steep wave front voltage is only applied across the first discharge gap G between the electrodes 1 and 2 to be discharged thereacross thereby to apply the voltage between the terminals A and C across the second discharging gap G between the electrodes 2 and 3 then the following electric discharge must be immediately developed across the second gap. However if the voltage across the terminals A and C is not of a certain high magnitude some interval of time will lapse until the second discharge gap is discharged. During this interval of time the applied voltage is raised so that it is dif ficult to decrease the sparkover voltage of the entire arrangement so much for the steep wave front voltage. As a time delay with which the gap structure is arced is great for SP gas the arrangement of FIG. 3 is difficult to greatly improve the discharge characteristics thereof. The results of experiments indicated that the arrangement of FIG. 3 could only decrease the sparkover voltage for any steep wave front voltage by the order of percent.

Also there is already known another means for improving the V-T characteristic of lightning arrester devices as illustrated in FIG. 4. The arrangement illustrated comprises a pair of main spherical electrodes 1 and 3 disposed in spaced opposite relationship to form a main discharge gap G therebetween, a triggering electrode 4 disposed adjacent one of the main electrodes, in this case, the electrode 3 to form a pair of longer and shorter discharge gaps therebetween, and a pair of voltage dividing impedance elements 2 and Z for connecting the triggering electrode4 to the main electrodes 1 and 3 respectively. When a voltage applied across a pair of terminals A and C connected to the main electrodes 1 and 3 respectively and therefore across the main discharge gap G has reached a predetermined magnitude, a very small triggering discharge is caused across the triggering and main electrodes 4 and 2 respectively by any suitable means (not shown) whereby the main gap G is discharged with no time delay.

The arrangement of FIG. 4 has been extremely effective for operating with the conventional arc-extinguishing medium formed of air or nitrogen. Many modifications of the arrangement as shown in FIG. 4 have been used and are being used for practical purposes.

However the arrangement of FIG. 4 has been found to be less effective for operating with a gas high in electrically negative property such as SF gas. For example, FIG. 5 used the axis of ordinates and abscissas are the same in meaning as those for FIG. 2 shows the results of experiments conducted with the arrangement such as shown in FIG. 4 including the main discharge gap 7 millimeter long and disposed in an atmosphere of each of nitrogen (N) under 3.22 absolute atmospheres and sulfur hexatluoride (SF.,) under 1.66 absolute atmospheres. Impulse voltages having a duration of 0.2

microsecond of wave front and a duration of 200 microseconds of wave tail were applied across terminals A and C connected to' the main electrodes 1 and 3. In FIG. 5 dots and broken line designate the case the triggering discharge was effected within 1 microsecond measured from the beginning of the wave front of the impulse voltage after its application across the terminals A and C while crosses and solid line designate the case no triggering discharge was effected. Also upper solid and broken curves describe the V-T characteristics for the SP gas and lower broken curve describes the V-T characteristic for the nitrogen.

From lower broken curve shown in FIG. 5 it is seen that for the atmosphere of nitrogen, the occurrence of the triggering discharge immediately causes a decrease in sparkover voltage across the main discharge gap while a time delay is scarcely found. On the other hand, upper curves shown in FIG. 5 depicts that for the atmosphere of SF gas the sparkover voltage due to the triggering discharge begins to decrease with a time delay amounting to 10 microseconds or more. Further the I decrease in sparkover voltage due to the triggering discharge to form negative ions. Those negative ions are accumulated in I the discharging gap during the particular discharge thereby to distort the associated electric field until the main gap is broken down at a low voltage although the formation the negative ions would not be directly linked with the breakdown of the main discharge gap. Therefore an interval of time required to accumulate the negative ions in the main gap leads I to a long time delay with which the electric discharge is initiated.

Thus it will be appreciated that if the triggering discharge previously used in combination with nitrogen or air is employed in gases high in electrically negative property such as SF gas that a transition time from the triggering discharge to the main discharge becomes very long with the result that the sparkover voltage for steep wave front cannot decrease.

The invention contemplates to eliminate the disadvantages of the prior art practice as above described by the provision of a first and a second main electrodes having a triggering electrode as a third electrode, disposed nearly midway therebetween.

FIG. 6 is a schematic view of a discharging gap structure illustrating the principles of the invention. The arrangement illustrated comprises a pair of main discharge electrodes 1 and 3 including semi-spherical end portions opposing to each other to form a main discharge gap therebetween, and a triggering electrode 4 including a pointed end portion nearly centrally extending into the main discharge gap and somewhat short of the common axis of the main electrodes to divide the main gap into a pair of discharging gaps G and G of nearly equal formed between the electrodes 4 and 3 and length between the electrodes 1 and 4. The electrodes 1 and 3 are connected to terminals A and C respectively. The triggering electrode 4 is connected to an intermediate terminal B and thence through voltage dividing elements 2 and 2 to the main electrodes 1 and 2. The voltage dividing element Z, and Z serially connected across the terminals A and C serve to divide a voltage applied across the terminals A and C as determined by a ratio of impedance of the element Z, to that of the element Z In the absence of the triggering electrode 4, the discharging gap defined by the main electrodes 1 and 3 has intrinsically such a discharge characteristic that the sparkover voltage is very higher for steep wave front voltages than for slow wave front voltages. In the presence of the triggering electrode 4, however, a voltage applied across the terminals A and C is divided by the voltage dividing elements Z and Z serially connected to each other and then applied across the gaps G and G, through the triggering electrode 4. The voltage dividing elements Z, and Z have the respective impedances preselected such that, upon applying a slow wave front voltage across the terminals A and C, the voltage is divided into two voltage portions approximately equal to the sparkover voltages across the discharging gaps G, and G, or across the electrodes 1 and 4 and across the electrodes 4 and 3. However, upon applying a steep wave front voltage across the terminals A and C, the voltage is divided into two unequal voltage portions such that the greater part of the voltage is applied across the discharging gap G, between the electrodes 4 and 3. Therefore the application of a steep wave front voltage across the terminals A and C causes the discharging gap G, between the electrodes 4 and 3 to be discharged first. Thereby when an overvoltage is applied across the discharging gap G between the electrodes 1 and 4 which cooperates with the triggering function resulting from the electric discharge across the gap G, to induce an electric discharge across the gap G without any time delay at that moment.

The invention is characterized in that the triggering discharge path forms a part of the main discharge path while the main discharge path yields a single electric arc without the triggering electrode 4 dividing the main discharge path.

While the position of the triggering electrode 4 disposed between the main electrodes 1 and 3 depends upon the selection of the voltage dividing elements Z, and 2,, a ratio of a sparkover voltage across the gap G, to that across the gap G is preferably selected to range from 0.5 to 2 in order to meet the following two requirements:

1. When a slow wave front voltage is applied across the terminals A and C and divided between the gaps G, and G as determined by a ratio of a discharge voltage across the gap G, to that across the gap G the sparkover voltage across the entire gap structure as above described should be substantially equal to the sparkover voltage across the discharging gap G, for any steep wave front voltage applied across the terminals A and C, and

2. in order that after a steep wave front voltage has been applied across the terminals A and C to discharge the discharging gap 0,, the discharging gap G is discharged without any time delay, the sparkover voltage across the discharging gap G should not be much different in magnitude from that across the discharging gap 6,.

In accordance with both the discharge characteristic of each of the discharging gaps G, and G and the V-T characteristic required for the entire gap structure, the impedances of the voltage dividing elements can be selected at will. Since slow wave front voltages actually applied across discharging gap structures have a variety of different waveforms it is required to select the voltage dividing elements Z, and Z so as to render the overall V-T characteristic flat in due consideration of the discharge characteristics of the gaps G, and G relative to those waveforms of the slow wave front voltages.

FIG. 7 wherein like reference characters designate the components identical or similar to those shown in FIG. 6 illustrate a discharge gap structure constructed in accordance with the principles of the invention and applied to a lightning arrester device including an arc-extinguishing medium formed of SF, gas. The arrangement illustrated is different from that shown in FIG. 6 in that in FIG. 7 the main electrodes are of needle type. The voltage dividing element Z, is formed of a capacitor 5 connected across the terminals A and B and the other element Z, isformed of a series combination of a capacitor 6 and a resistor 7 connected across the terminals B and C and having a resistor 8 connected across the capacitor and resistor combination.

It is assumed that the capacitors 5 and 6 have capacitances of 160 and picofarads respectively and the resistors 7 and 8 have the magnitudes of resistance of 100 kilohms and 40 megohms respectively while a voltage is applied across the terminals A and C having a magnitude linearly increased with time such as shown at waveform E, in FIG. 1. Under the assumed condition, a ratio of the applied voltage E, to a voltage applied across the element 2, was calculated. The result of the calculation is illustrated in FIG. 8 wherein the axis of ordinates represents time in microseconds. From FIG. 8 it is seen that the voltage ratio as determined by the voltage dividing elements Z, and 2, does not depend upon the steepness of the applied voltage and instead is a function of time alone. Also it is seen that substantially the entire magnitude of the voltage is applied across the discharge gap G, for steep wave front voltage having the duration of wave front of l microsecond or less while about 40 percent of the entire magnitude of the voltage is only applied to the gap G, for slow wave front voltages having the duration of wave front of 100 microseconds or more.

In FIG. 9 wherein the axis of ordinates represents a sparkover voltage in kilovolts and the axis of abscissas represents time in microseconds, curve L, describes the V-T characteristic of the discharge gap G, coupled to the voltage dividing elements Z, and Z having the magnitudes as above specified and curve L depicts the V-T characteristic between the terminals A and C having no voltage dividing element connected thereacross. The electrode system having the V-T characteristics such as shown at curves L, and L has been determined to have the overall V-T characteristic flat as shown at curve L in FIG. 9. It will be appreciated that the V-T characteristic shown at curve L, can be changed at will by varying the magnitudes of the capacitors 5 and 6 and the resistors 7 and 8. From the elementary circuit theory it can readily be conceived that a number of circuit parameters such resistances, capacitances and inductances may be suitably interconnected in series and parallel to impart to the voltage dividing elements Z, and Z, a voltage dividing characteristic as desired.

While the invention has been described in conjunction with such a voltage division that for steep wave front voltages almost the entire magnitude of the voltage across the main electrodes is first applied across the discharge gap G, it is to be understood that it is in the scope of the invention to divide a steep wave front voltage so as to apply a small fraction of the voltage across the discharge gap G simultaneously with the application of the remaining portion of the voltage across the discharge gap G,. The later voltage division may become rather advantageous because the occurrence of the electric discharge across the discharging gap G, causes an increase in voltage applied across the discharging gap G to permit the latter gap to be discharged with a shorter time delay.

In summary, the invention includes a pair of main discharge electrodes disposed in spaced opposite relationship and a triggering electrode disposed between the main discharge electrodes at such a position that it defines a pair of discharge gaps with the pair of main discharge electrodes and still permits a main single electric arc to be developed between the main discharge electrodes upon initiating the electric discharge therebetween while the main electric arc is not divided by the triggering electrode and that, upon applying across the pair of discharge gaps voltages equal in waveform to each other, a ratio of a sparkover voltage across one of the discharge gaps to that across the other discharge gap ranges from 0.5 to 2.0.

The invention has several advantages. In lightning arrester devices including the arc-extinguishing medium formed of an electrically negative gas such as SF gas and means for magnetically driving electric arcs developed across the main discharge electrode to extinguish them, it is desirable to make the length of the electric are as long as possible for the purpose of reducing the sticking time of the electric arc. In this respect the invention is advantageous in that as the triggering electrode does not divide electric arcs formed between the main discharge electrodes, the discharge gap defined by the main electrodes can increase in length as compared with the conventional discharge gap structures including a pair of discharge gaps interconnected in series relationship. This results in a reduction in sticking time and therefore an increase in interrupting characteristic. Also the sparkover voltage remains substantially unchanged in magnitude even for different waveforms of voltage applied across the main discharge electrodes. Further the voltage dividing elements connected in series to each other across the discharge gaps may vary in impedance to obtain any desired V-T characteristic.

What we claim is:

1. For use in a lightning arrester device including, an arcextinguishing medium formed of an electrically negative gas, a discharging gap structure comprising, a pair of main discharge electrodes disposed in spaced opposed relationship and a thin triggering electrode coupled to said main discharge electrodes by said arc-extinguishing medium disposed between said main discharge electrodes to define a 'pair of discharging gaps with said main discharge electrodes, said triggering electrode being located at a position that, upon applying voltages of the same waveform across said discharging gaps respectively, a ratio of a sparkover voltage across one of said discharging gaps to that across the other gap ranges from 0.5 to 2.0, said triggering electrode permits said main discharge electrodes to form a main single electric arc therebetween, and said triggering electrode being of a length so that it does not extend into the path of the axis of minimum spacing between said main discharge electrodes.

2. A discharging gap structure as claimed in claim 1, wherein a pair of voltage dividing elements connected in series to each other and across said discharging gaps respectively and have impedances such that a steep wave front voltage is substantially entirely applied across one of said discharging gaps while a slow front wave voltage is divided into a pair of voltage portions as determined by the ratio of the sparkover voltage across said one discharging gap to that across the other gap, and said pair of voltage portions being applied in operation across said discharging gaps respectively. 

1. For use in a lightning arrester device including, an arcextinguishing medium formed of an electrically negative gas, a discharging gap structure comprising, a pair of main discharge electrodes disposed in spaced opposed relationship and a thin triggering electrode coupled to said main discharge electrodes by said arc-extinguishing medium disposed between said main discharge electrodes to define a pair of discharging gaps with said main discharge electrodes, said triggering electrode being located at a position that, upon applying voltages of the same waveform across said discharging gaps respectively, a ratio of a sparkover voltage across one of said discharging gaps to that across the other gap ranges from 0.5 to 2.0, said triggering electrode permits said main discharge electrodes to form a main single electric arc therebetween, and said triggering electrode being of a length so that it does not extend into the path of the axis of minimum spacing between said main discharge electrodes.
 2. A discharging gap structure as claimed in claim 1, wherein a pair of voltage dividing elements connected in series to each other and across said discharging gaps respectively and have impedances such that a steep wave front voltage is substantially entirely applied across one of said discharging gaps while a slow front wave voltage is divided into a pair of voltage portions as determined by the ratio of the sparkover voltage across said one discharging gap to that across the other gap, and said pair of voltage portions being applied in operation across said discharging gaps respectively. 